Ron made a good point about Sen timing (Sen no Sen is another). Aikido is all about taking and controlling the initiative imo. It's not a solely "defensive" martial art - that is a total fallacy.

On another point, I think some folks are confusing speed with deception. Most folks who get stabbed without knowing it or being able to react quickly enough tend to end up in that situation not because they saw a knife and were slow to react, avoid, defend themselves or attack, it is because they never knew a knife was part of the engagement to begin with. The stab tends to be the point where the victim first suspects that a knife is involved.

Having done some training in tactical folding knife (as taught to SWAT and those types), a lot of the training is based on the premise that you never let the target see the knife until after he feels it penetrating a vital organ, artery or muscle. It's all about a calm, deceptive, sharp, focussed surprise attack.

So imo it is not so much about speed, though it does take much less movement to seriously and fatally wound with a knife than it does to avoid being stabbed, effectively block or control the attacker. The main thing is that someone trained to kill with a knife will not allow it to be seen. If you can see it before it is in you, then you may stand a chance to do something and escape.